1. Field of the Invention
The invention relates to a process and an apparatus for the fermentation of carbohydrate-containing nutrient substrates, particularly of molasses, in the preparation of citric acid. More particulaly, the invention relates to a process and apparatus for the inoculation of a fermentation solution with a fermentation agent in a sterile fermentation chamber, forming in a germination phase, a layer of the fermentation agent covering the entire surface of the fermentation solution, and then converting the sugar into citric acid, with the addition of air, in a fermentation phase.
2. Description of the Prior Art
Since the discovery of the citric acid forming capability of certain mould fungi; primarily aspergillus types, carbohydrate-containing nutrient substrates have been known to be suitable to undergo a fermentation process for the technical preparation of citric acid. Several substances such as sugar-containing products, starch, and starch-containing raw materials after sachharification are used as the nutrient substrate. Of these nutrient substrates, molasses has been an especially suitable substrate for fermentation. In the conventional processes, the liquid nutrient substrate which is to be fermented is first brought to a sugar content of 12 to 20 percent and is enriched with nutrient salts and possibly with fermentation-promoting additives. The enriched nutrient substrate is then sterilized, following the adjustment to the most favorable pH value. The sterilized fermentation substrate is fed from the cooker, through sterile lines, into flat pans, designated fermenting vessels, which are arranged on top of each other in the so-called fermenting room. Prior to filling, the vessels and the chambers must be sterilized with a disinfectant. All measures which are required for a sterile fermentation must be taken with special care because, otherwise, sensitive disorders through infections, penicillium types, yeasts, or bacteria are unavoidable. This fact represents a considerable disadvantage in the present practice of fermenting carbohydrate-containing nutrient substrates.
After the filling of the fermentation solution into the vessels, inoculation with spores of the micro-organism takes place. During the germination phase, usually about two days there is no heat development because citric acid is not yet generated during this time. A continuous mould cover is, however, formed during this period. It is widely recognized, therefore, that the vessels must be subjected to an external source of heat during germination. Without the supplemental addition of heat, the fermentation solution would be cooled, due to radiation, to the point that the temperature of the fermentation solution would be insufficient to support the germination. A small amount of oxygen is also required during this phase of the procedure.
Up to now, the loss of heat by radiation during the germination phase is compensated for by the introduction of large quantities of conditioned air, i.e., tempered and moistened, if required. Disadvantages of this technique arise from the required sterility of the blown-in air. This problem is further compounded by the large quantity of heated air which is necessary to prevent dropping of the temperature of the fermentation solution into ranges which prevent successful fermentation. Accordingly, the requirement of introducing these large quantities of air into the chambers without affecting the sterile environment can be fulfilled only with considerable effort. The requirement for a high degree of sterility is particularly important for the germination phase to prevent the reduction of yields, and in some cases, even total failure. Therefore, when employing such large quantities of air as heat carriers, it is not certain that infections would not also be carried into the chambers with the large quantity of air, especially in view of the size of the air conditioning system for the tempering and moistening, and the complexity of the associated filters.
After a good development of the mould cover during the germination phase, the citric acid formation starts and the so-called fermentation phase begins. It is characterized by the fact that, in the conversion of the sugar into citric acid, large quantities of heat are liberated, which must be removed from the chambers. The total process takes about 7 to 11 days. About 60 to 90 percent of the originally available sugar is converted into citric acid. The fermentation liquid is then drawn from the vessels and fed into vats in which the acid is precipitated as calcium citrate with the aid of milk of lime. The mould cover is washed and pressed and the wash water is combined with the concentrated citric acid solution in the precipitation vat. In accordance with the existing conventional processes, the heating air, required during the germination phase, as well as the cooling air, which takes care of the removal of the heat during the fermentation phase, are simply fed into the chambers through a single combined heating/cooling system.
In summary, this practice has been found to have a negative effect on the operating reliability of the overall process. Because the air which is fed into the chamber as heat carrier during the germination phase is used to maintain the temperature of the fermentation solution, and the quantities must therefore be correspondingly large, infections cannot be avoided, in spite of all efforts to assure the sterility of the air. The control of the fermentation solution temperature is carried out through a complicated control system with multiple variable dependent functions, in which both the quantities of air and also their temperatures and moistures are adjusted in accordance with the required fermentation climate.
The air, which is fed into the chamber during the germination phase, must be especially well filtered and even sterilized because of the increased susceptibility of the fermentation solution to infections; this means the application of considerable resources in view of the quantities of air which are used. In case of failure of one of the above-mentioned necessary conditions for carrying out the process, the result can be infections or undercooling of the fermenatation solution, which is associated with a poor fermentation yield or loss of a chamber.